Transformer coupling circuits



Aug. 23, 1960 R. w. SPENCER TRANSFORMER COUPLING CIRCUITS 2 Sheets-Sheet 1 Filed Sept. 26, 1956 (PRIOR ART) FIG.

INVENTOR.

Richard W Spencer FIG (2.

1950 R. w. SPENCER 2,950,441

TRANSFORMER COUPLING CIRCUITS 2 Sheets-Sheet 2 Filed Sept. 26, 1956 IN VEN TOR.

Richard W. Spencer 2,950,441 TRANShGRD/ER CGUPLING CZRCUITS Richard W. Spencer, fhiladelphia, Pa, assignor to Sperry Rand Corporation, New York, N.Y., a corporation of Delaware Filed Sept. 26, 1956, Ser. No. 612,306 3 Claims. (Cl. 328-221) The present invention relates to improved coupling arrangements for use in signal responsive control circuits; and is more particularly concerned with an improved arrangement for providing transformer coupling in such circuits.

Various circuit arrangements utilized in control applications are known wherein a transformer coupling element is interposed between a source of driving pulses and a utilization circuit. One particular circuit, generally of this configuration, is often employed in computing and recording applications wherein unidirectional pulses of voltage on one or the other (but not both simultaneously) of two inputs, are employed for controlling write amplifiers; and a typical such circuit of this general type may take the form of a pair of driving magnetic amplifiers coupled to opposing ends of a trans former primary winding, with the opposing ends of the transformer secondary winding being in turn coupled to the control grids of vacuum tube elements.

Typical circuits of the type described above ordinarily include center tap connections for both the primary and secondary windings of the aforementioned coupling transformer; and it has been the practice in the past to couple the primary winding center cap to a source of reference potential which is greater than or equal to one half the maximum signal input which may be expected. Similarly, it has been the practice in the past, particularly when vacuum tube utilization circuits are employed in combination with the transformer secondary winding, to couple the center tap of such a secondary winding to a further source of reference potential having a magnitude substantially equal to the negative cutoff potential of the said vacuum tubes.

Such arrangements have been subject to a number of disadvantages which will become more apparent from the subsequent description; but, in general, it may be said that due to the aforementioned reference sources utilized in conjunction with the transformer primary and secondary winding center taps, inherent variations in applied signal at the transformer primary winding have been ag gravated at the circuit output; and in addition, power has often been unnecessarily wasted due to the charging and discharging of stray capacitances, particularly those associated with the vacuum tube grids in the secondary circuit.

The present invention serves to obviate these difficulties in known transformer coupling circuits and contemplates the provision of improved coupling circuits wherein there is less variation of signal amplitude at the circuit output for a given variation at the circuit input than has been the case heretofore. The circuits to be described also efiect lower capacitive loading, as seen at the circuit input, due to load circuit capacitance; provide for possible elimination of at least one voltage source considered necessary heretofore; and in addition, permit the use of a coupling transformer having fewer turns on the secondary winding for a given signal than has been the case ieretofore.

it is accordingly an object of the present invention to provide improved transformer coupling circuits.

A further object of the present invention resides in the provision of improved circuit configurations for use 2 @50 44 use 9 Patented Aug. 2? 1 5ml in conjunction with either the primary winding, secondary winding, or with both primary and secondary windings of a coupling transformer.

Another object of the present invention resides in the provision of a transformer coupling circuit which is more efficient in operation and which provides for lower capacitive loading due to stray circuit capacitances than has been the case heretofore.

A still further object of the present invention resides in the provision of a transformer coupling circuit having a smaller variation of signal amplitude at the circuit output for a given variation at the circuit input than has been the case in the past.

A further object of the present invention resides in the provision of an improved transformer coupling circuit which requires fewer turns on at least the secondary winding of the said transformer than has been the case heretofore, whereby the coupling circuit itself may be made in smaller sizes and at less expense than has been the case in the past.

Another object of the present invention resides in the provision of an improved transformer coupling circuit which permits the elimination of at least one voltage source considered necessary heretofore, whereby the overall arrangement is less expensive and may be included in more complex circuits with greater facility than has been the case heretofore.

In providing for the foregoing objects and advantages, the present invention contemplates the provision of a transformer coupling circuit having primary and secondary windings which are center tapped, as has been the case in the past. Rather than coupling these center taps to the sources of reference potential which have been utilized heretofore, however, the center tap connections of the said coupling transformer are allowed to float in potential and are returned via resistors or equivalent impedance elements to reference sources, for instance such as ground, or such as the cutoff potential of vacuum tube utilization circuits which might be employed.

in addition, clamping elements are provided in association with the control grids, for instance, of vacuum tube utilization circuits coupled to the transformer secondary, thereby to prevent these control grids from falling below their predetermined cutofi potential, whereby power is not unnecessarily wasted in the charging and discharging of stray capacitances in the secondary circuit. Similar clamp means may optionally be utilized in conjunction with the primary circuit, if desired, to prevent input terminals associated with that primary circuit from falling below some preselected reference potential such as ground.

The overall arrangement thus provided effects substantially improved operation of the coupling circuit and of circuits associated therewith, as will be described subsequently in greater detail.

The foregoing objects, advantages, construction and operation of the present invention will become more readily apparent from the subsequent description and ac companying drawings, in which:

Figure l is a schematic diagram of a typical transformer coupling circuit of the type utilized heretofore; and is provided to facilitate an explanation of the problems and undesirable circuit characteristics associated with such prior coupling devices.

Figure 2 (A through F) are waveform diagrams illustrating typical operation of the circuit shown in Figure I- Figure 3 is a schematic diagram of an improved transformer coupling circuit constructed in accordance with one embodiment of the present invention; and

Figure 4 (A through F) are waveform diagrams illustrating typical operation of the circuit shown in Figure 3.

Referring now to Figure 1, it will be seen that, in accordance with the present invention, a transformer coupling circuit may comprise a transformer T having a primary winding llland a secondary winding 11. The op-.

posed ends of primary winding may be coupled via rectifi'eis D1 and D2 to signal input terminals 12 and 13 respectively; and similarly, the opposing ends of secondary winding 11 may be coupled to the control grids so selected that it is equal to orgreater than one-half.

of the maximum signal amplitude (E max.) which may be expected at either of input terminals 12 or 13. Similarly, in accordance with known arrangements, the center tap of secondary winding 11 is ordinarily coupled to a further reference source 21 having a magnitude substantially equal to the cutoff potential of the tubes 14 and 15. a e

v The typical operation of a circuit of the type illustrated in Figure '1 may be more readily apprecated by reference to the waveform diagrams in Figure 2 (A through F). In'a circuit of the type described, it is ordinarily the practice to provide unidirectional pulses of signal voltage atone or the other (but not both simultaneously) of the input terminals 12 and 13. Such signal pulses, occurring during mutually exclusive input times, have been designated respectively as e and e and are illustrated in Figures 2A and 2B. In ea ch of these figures, as well as in the otherwaveform diagrams in Figures 2 and 4, to be described, it will be noted that both a full line and dotted line representation has been employed in association with each voltage excursion. The full line representation may be termed E min. and represents the minimum signal voltage which may be expected at an input terminal, or at some other: point on the circuit due to circuit tolerances; and similarly, the dotted linerepresentation which has been termed E max. represents the maximum voltage excursion which may be' expected at a given input terminal or at other circuit points, again due to various circuit tolerances. a

It will be appreciated that due to the possible variation in signal amplitude whichmay occur at either'o'f the input points 12 or 13, a further variation in the possible range of potentials will be present at both primary winding 10.and at secondary winding 11. In the particular circuit shown in Figure 1 the center mp 18 is, as has been described, returned to a voltage source +V having a magnitude equal to or greater than /2E max, and this particular magnitude of +V has been selected in the past to assure that when a positive voltage pulse, for instance, is applied to terminal 12, the total voltage developed across primary. winding 10 will cause the lower end of the said primary winding (see potential e to still r be of such value that rectifier D2 will not conduct (see Figures 2C and 2D). This choice of potential magnitude for source +V 'of course, causes a portion of inputvoltage, equal'to /2E max., to be wasted in overcoming the bias.pro.vided by source 19; and the situation is accordingly aggravated when a signal voltage less than E max..is applied to one or the other of terminals 12 or 13.

' This will be more readily appreciated from the following.

'If we should assume that 13 max. .is applied for in stance to terminal 12, the said potential E max. will .be developed across? primary winding 10 whereby the potential between center tap 18 and theu pper end of the said primary winding is E max. minus /2E 111311., or /2E max. On the other hand, if a signal of magnitude E min'. should be applied, for instance to terminal 12, the

potentialidevelopediacross transformer winding '11), between center tap 18 and the upper end of the said-windf r be effectively cut oh by a potential of -E This driving, is equal to E min. minus /2E max., whereby the actual potential appearing across the said upper half of substantially equal to the cutofi potential of tubes 14 and 15. When a signal is applied to one or the other of input terminals 12 or 13, this signal, of course, is coupled via transformer T to the secondary winding 11 and serves to drive the control grid of one or the other of tubes 14 and 15 positive with respect to its cutofi potential -E At the same time, however (see Figures 2E and 2F), the other grid is driven below a potential of E even though the tube not driven into'conduction would ing of a non-conducting output tube below a control grid potential equal to its cutoff value, is actually accompanied by a charge and discharge of stray capacitance in the secondary circuitand results in an unnecessary waste of power in the said secondary circuit. The foregoing considerations, as well as the'voltage excursions occurring at various. points in the circuit, will be readily appreciated from the waveforms given in Figures Av through F of Figure 2. These waveforms have been labelled e through 2 inclusive, and the corresponding circuit points V have been designated in Figure 1.

The undesirable operational characteristics thus in-.

herent in prior transformer coupling circuits are obviated in accordance with the present'invention byan improved coupling arrangement generally of the type shown in Figure 3; and those element's of Figure 3 which correspond to elements already described in reference to-Fig urel have been given likenumerals. Thus, referring to Figure 3, itwill be seen that, as. before,a transformer T may have a primary winding 10 center-tapped at point 18, and the opposing ends on said winding 10 are coupled via rectifiers D1 and D2 to input terminals 12 and 13. Similarly, the transformer T has a secondary winding 11 center tapped at point 20, and the opposing ends thereof 'are'coupled to the control grids of vacuum tubes 14 'of ground potential 30. Similarly, rather than directly coupling the center tap 20 of. secondary winding 11 to a source of potential 21 having a magnitude E the said center tap 20 is now coupled to such a source 21 via a further resistive element R "In addition, clamp diodes D3 and D4 are coupled, as shown, between the control grids of tubes 14 and 15 and the negative reference source 21 (E 'to prevent the said control grids of tubes 14 and 15 from falling below their cuto'if potential 'E As will. become apparent from' the subsequent de scription, the resistor R1 associated with center tap 18 in the primary, circuit, need not. be coupled to ground but can in fact be'coupled to any reference source having apotential smaller in magnitude than min.;. and the grounded representation at terminal 30, while adopted for convenience in bothdiscussion and in circuit 'con struction, is accordingly optional. Similarly, resistor R2, rather than returning center tap 20 in the'scco'ndary circuit to a source E can intact return th'e'said secondary to a source having a magnitude morjnegative 7 than E Each of these optional casesmaytbe readily R1 and R2. In either event, however, the control grids of tubes 14 and 15 should be clamped at E as illustrated in Figure 3; and similarly, the input terminals 12 and 13 may be clamped at ground potential, if desired.

It should further be noted that while the arrangement of Figure 3 shows an improved center tap circuit connection in association with both the primary and secondary windings, the primary circuit connection shown in Figure 3 may, if desired, be utilized in combination with a secondary connection of the type shown in Figure 1, wherein the overall circuit acts to minimize variation in signal output for a given variation in signal input. Similarly, the secondary circuit shown in Figure 3 may be associated with a primary circuit of the type shown in Figure 1, wherein the overall circuit acts to reduce the wasting of power efi'ected through charge and discharge of stray capacitance in the said secondary circuit. A preferred form of the invention, however, is in fact illustrated in Figure 3, wherein the improvements of the present invention are associated with both the primary and secondary windings, thereby to accomplish both of these advantageous results.

The operation of the circuit shown in Figure 3 will become readily apparent from an examination of the waveforms of Figure 4 (A through F); and again, the various potentials illustrated in Figure 4 have been correspondin ly designated in the circuit of Figure 3. In particular, it will be noted that Figures 4A and 4B illustrate the application of unidirectional voltage pulses e and e at one or the other of input terminals '12 or 13, and in this respect, therefore, Figures 4A and 4B correspond to Figures 2A and 2B. Due to the floating nature of the center tap 18 in the primary circuit, the potentials appearing at the opposing ends of primary winding 13 (and designated 2 and e.,, respectively), are now selectively positive-going from ground potential (for the particular connection of point 30), rather than from the reference potential +V- illustrated in Figures 2C and 2D. The general waveforms for 2 and 2 are also illustrated in Figures 4A and 4B.

As mentioned, the potential (e of center tap 13 in the primary circuit, is now floating in nature rather than being fixed at the value +J as was the case in Figure 1; and the actual potential at center tap 1.8 will in fact be equal to one-half the potential of signal pulse appearing at either terminal 12 or terminal 13. Thus, when E max. is coupled to either terminal 12 or 13, the potential at center tap 18 rises to /2B max. whereby a potential of /2B max. is developed across one-half the primary winding. This, of course, corresponds to the case of E max. in Figure 1. When E min. is applied to terminals 12 or 13 in the arrangement of Figure 3, however, center tap 18 rises to a potential equal to /2B min. whereby again one-half of the actual applied potential is developed across half of the primary winding. Thus, due to the floating nature of center tap 18, the ratio of potentials E max/E min., appearing across primary winding 10 or across any portion thereof, remains identical with the ratio appearing at either of terminals =12 or 13; and this in turn represents a significant reduction in possible voltage variation over coupling circuits utilized heretofore.

The center tap 20 associated with secondary winding 11, is similarly floating in nature; and this floating characteristic of center tap 26 provides a further significant advantage. Thus, returning for a moment to the arrangement of Figure 1, it should be noted that inasmuch as center tap 29 thereof is returned directly to a potential E a total potential of 2E would have to be developed across secondary winding 11 of Figure 1 in order to drive the control grid of one or the other of vacuum tubes 14 and 15 to zero bias. Due to the floating arrangement of center tap 20 in Figure 3, however, a total potential of E developed across the entire secondary winding '11, will now serve to drive the control grid of one or the other of vacuum tubes 14 and 15 to zero bias; and as a result, the number of turns on the secondary winding of Figure 3 may be reduced, and in fact may be halved, in comparison with the number of turns required in the secondary winding of the arrangement shown in Figure 1. In addition, it will be appreciated that due to the provision of clamp diodes D3 and D4 in the secondary circuit of Figure 3, the control grids of tubes 14 and 15 are clamped at a potential -E and cannot fall below that potential, whereby waste of power in the charge and discharge of stray circuit capacitances is avoided.

The general operation of the circuit illustrated in Figure 3 will be appreciated from an examination of the several waveforms shown in Figure 4 (A through F), and again, these waveforms carry designations corresponding to those given in Figure 3.

While I have thus described preferred embodiments of the present invention, it must be stressed that the foregoing discussion is meant to be illustrative only and should not be considered limitative of my invention. Thus, for instance, the output vacuum tube stages 14 and 15 may take other forms, including transistor stages. If PNP type transistors are to be employed, for instance, each of the vacuum tube stages in the circuit of Figure 2 may be replaced by a transistor having its emitter grounded and having its base connected to one of the ends of secondary winding 11, in which event outputs could be taken at the transistor collector. In such a case, the polarity of source 21 would have to be positive rather than negative. Other forms of transistors, and other connections for such transistors may also be employed, of course, so long as proper attention is given to the polarities of the various sources already discussed.

Still further variations will be suggested to those skilled in the art, and certain of these variations have in fact been discussed. All such variations and modifications as are in accord with the principles described are accordingly meant to fall within the scope of the appended claims.

Having thus described my invention, I claim:

1. In a coupling circuit, the combination of a transformer having a primary winding and a secondary winding, means selectively applying unidirectional control pulses to opposite ends of said primary winding, separate utilization means each having a first electrode coupled to opposite ends of said secondary winding, a source of reference potential having one terminal connected to another electrode of each of said utilization means, impedance means operable to maintain said first electrode of said utilization means at the potential of the other terminal of said source in the absence of an input signal, and unidirectional current conducting means coupled between said other terminal and each of said opposed ends of said secondary winding, said unidirectional current conducting means being poled to allow easy current flow from said other terminal to said opposed ends whereby the potential induced in said secondary winding between said opposed ends is effective to bias either of said first electrodes to a potential above that of said other terminal equal to the potential induced between the opposed ends of said secondary in response to said control pulses.

2. The combination of claim 1 wherein said impedance means is connected between said other terminal and a center tap of said secondary.

3. The combination of claim 1 wherein said utilization means are vacuum tubes, said first electrodes are the control grids of said vacuum tubes, and said other electrodes are the cathodes of said vacuum tubes.

References Cited in the file of this patent UNITED STATES PATENTS 

